Count incorrect in MongoDB

Question

Tech:

• MongoDB 3.0.8 (MMAPv1), not sharded, dedicated cluster hosted on AWS via mLab, (primary, secondary and arbiter), 3.7GB RAM
• C# driver for MongoDB 2.3
• Connection string:

mongodb://USER:PASS@MYMONGO1.com:1234,MYMONGO2.com:1234/DB_NAME?replicaSet=REPLICA_SET_NAME

Assumptions

• I have collection Products with one of the fields called Package.
• There is index: "Package": 1
• Objects from this collection are never being deleted.
• The field Package is never being updated.
• New objects are inserted from time to time.

Once a day I log a specific count on this collection (same parameters every time):

db.Products({"Package": "Box"}).count()
// actual code running in C#:
productsCollection.Find(p => p.Package == "Box").Count()

I expect the result to be the same or greater every day. But sometimes I get a smaller value. The next day it becomes correct again. It reproduced on two different environments.

Example:

• Day 1: 4,563,135
• Day 2: 4,563,135
• Day 3: 4,563,124 (exactly 11 less than expected)
• Day 4: 4,563,135

I was trying to manually reproduce it both via C# and directly against Mongo, but failed (the value was always correct).

What's going on?

Answer 1

This is probably due to balancing round was taking place when you code was executed. From MongoDB documentation:

On a sharded cluster, db.collection.count() can result in an inaccurate count if orphaned documents exist or if a chunk migration is in progress.

More about it here: MongoDB documentation

To get the exact result one should use the aggregation framework queries